1. Field of Invention
This invention relates to paper coating compositions. More particularly, it relates to paper and paper board coating compositions containing smectite clays and starch which improve the performance of the paper and paper boards.
2. Prior Art
Chemicals, additives and polymers are often added to paper and paper boards in order to improve their performance. In order to obtain high quality paper it is necessary that the surface of the paper be smooth and have uniform porosity. Smooth and uniform paper is a prerequisite for good images printed thereon and also for good transfer of ink to the paper.
Smooth paper is conventionally obtained by coating the raw paper surface with a pigment composition. Coating compositions which create a smooth surface on paper have generally been aqueous dispersions, comprising mainly mineral fillers or pigments, such as Kaolin clay, calcium carbonate, and titanium oxide along with pigment binders of natural proteins, for example, casein or soy protein, starch or synthetic polymer emulsions. Coating compositions are usually applied to a continuous web of paper by high speed coating machines, such as blade coaters, air knife coaters, rod coaters and roll coaters.
The flow properties or runnability of coating compositions for paper and paper boards are of significant importance. These flow properties are often controlled by a thickener or co-binder.
The most common polymer used for the coating of paper is starch, preferably a modified starch such as hydroxyethylated starch. These modified starches can be augmented with other additives such as strengthening aids or hydrophobes. An example of a bulk strengthening aid, other than starch, is polyvinyl alcohol. Examples of hydrophobes or water repellant crosslinking agents are styrene acrylic polymers and melamine formaldehyde resins.
The clays conventionally used for pigments are preferably kaolinite or similar clays which generally has a relatively low surface area of about 10 to 25 m.sup.2 /g, relatively low viscosity of less than 100 centipoise at 10% solids and a relatively low aspect ratio of less than 50 to 1. (Aspect ratio is generally recognized in the industry as the ratio of the diameter to the thickness of the particular particle.) These clays, which are conventionally used as pigments or fillers, are generally used in pure form and are easily dispersible in water. For best results, these clays are used in slurry form with high clay concentrations of at least about 40 percent, with the lowest reasonable viscosity, preferably less than 100 centipoise.
Starch has also been used in these pigment coating compositions. In these compositions the clay is used as the pigment with the starch forming the adhesion material for adhering the clay pigment to the paper. For example, U.S. Pat. No. 3,607,331 discloses a paper coating composition comprising 30 to 50 percent clay, 5 to 7 percent a water soluble starch, 35 to 55 percent water and a starch insolubilizing agent.
European Patent Application No. 283 300 discloses a method of producing a coated paper suitable for gravure printing using a clay pigment. The printing process utilized does not require the paper to have high surface strength. The pigment is comprised predominantly of a water swellable, smectite-type clay. The suspension applied to the paper for use as a pigment contains up to 20 percent by weight of the pigment. In some of the Examples in the application, the smectite clay was merely slurried with water and then applied as a pigment to the surface of the paper. In Examples 3 and 4, a paper coating adhesive was added to the clay to assist in the binding of the smectite clay to the paper surface. In Example 3 the adhesive product was a latex of an acrylic copolymer, while in Example 4 a starch paper coating adhesive was used, wherein up to three times as much starch by weight as bentonite clay was used to secure it to the paper.
U.S. Pat. No. 3,869,296 discloses a paper pigment composition, wherein starch is utilized to adhere the pigment, such as a kaolin clay, to the paper, wherein a latent water insolubilizer for the starch is also added to the coating composition. Preferably, 100 parts of clay are combined with 20 parts of starch in the coating composition.
U.S. Pat. Nos. 5,283,129 and 5,494,509 disclose pigment coating compositions, wherein various materials are added to the clay/starch composition to enhance the quality of the paper. In each of these patents the starch is utilized as the binding agent for the clay, while the clay forms the pigment for the coating. In U.S. Pat. No. 5,283,129 the ratio of the clay to the starch binder is from about 75 up to 90 parts clay pigment to about 15 to 30 parts starch binder. In U.S. Pat. No. 5,494,509 the clay pigment portion comprises approximately 60 percent of the pigment slurry.
A binder complex, such as a colloidal silicic acid or cationic starch, is also commonly used to bind a filler to paper wherein the filler may be comprised of a kaolin, bentonite, titanium oxide, chalk or talc in U.S. Pat. Nos. 4,388,150, 4,385,961 and 5,071,512. See also U.S. Pat. Nos. 2,795,545 and 4,210,490.
Starch and clay products, such as bentonite clay, have also been utilized with cellulose fibers in the formation of paper and pulp sheets. For example, U.S. Pat. No. 4,210,490 discloses the combination of a cationic starch with a kaolin clay filler mixed with an aqueous solution of cellulosic fibers to form a paper product. See also U.S. Pat. No. 5,277,764 which discloses the utilization of a highly cationized starch for the formation of paper. See also U.S. Pat. No. 5,126,014.
When starch is used in paper coatings, but not as a binder to bind pigments to the paper, its primary role is twofold: (1) to penetrate into the sheet of paper in order to provide bulk strength and (2) to remain on the surface of the paper to improve surface strength, control porosity and absorbency. These two roles are in conflict as improvements in surface strength, porosity and absorbency require the starch to remain on the surface while improvements in bulk strength are achieved only through penetration of the starch into the paper sheet. Controlling the penetration of the starch into the paper sheet is thus necessary to balance these two characteristics and thus achieve optimum performance for the paper product.
In addition to controlling the extent of the penetration of the starch into the paper, enhanced quality for the paper products can be achieved by controlling the retrogradation of the starch product. Retrogradation, which is the association of starch chains, results in an increase in viscosity of a starch solution as that solution is cooled. Retrogradation problems are particularly present with unmodified starches, such as pearl starch, while they are less of a problem with modified starches, such as hydroxyethylated starches and particularly cationic starches. Being able to control the extent of penetration of the starch into a base sheet of paper and at the same time being able to stabilize the viscosity of the starch (retrogradation) would offer definite advantages in the production of paper.
Therefore, it is an object of this invention to produce a surface modifier for paper comprised of a smectite clay and starch which will both control the penetration of the starch into the paper and reduce its rate of retrogradation.
It is a further object of this invention to produce a coating for paper which will decrease the porosity of the sheets.
It is a still further object of this invention to produce a coating for paper with improved printing characteristics.
It is a still further object of this invention to provide a precoat for paper and boards to prevent a final coating from migrating into the sheet.
It is a still further object of this invention to produce a coating for paper comprised of a smectite clay and starch which does not require the use of a cationic starch.
It is a further object of this invention to produce a coating for paper with high viscosity at low solids and a very high aspect ratio.
These and other objects and features of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description and claims. The description provides selected examples of the preferred embodiment of the invention to illustrate the invention.